This invention relates to a method and apparatus for polishing the edge of a bonded wafer. More particularly, this invention relates to a method and apparatus for polishing the edge of a bonded wafer after reducing the diameter of the top wafer.
As is known, wafers for the semiconductor industry have been bonded together by a bonding layer in a sandwich type relation for various applications. Further, for these applications, the top wafer diameter needs to be reduced while the diameter of the bottom or handling wafer remains unchanged.
In cases where the bonding layer does not extend to the outer diameter, the top wafer is thinned down to microns of thickness and the unsupported area beyond the bonding layer becomes problematic. The thinning process tends to chip the unsupported wafer edge and the debris created causes scratching and subsurface damage on the prime surface of the wafer. A more extreme example of the damage is that large pieces of the top wafer break off with the bonding layer attached.
A partial solution to this problem is to reduce the diameter of the top wafer by using a fixed abrasive grind wheel on a conventional edge grinder. Such applications are currently performed by companies producing SOI wafers (silicon on insulator) and result in a ledge being formed along the periphery of the handling wafer while the diameter of the upper wafer is reduced. Typically, the depth of sub-surface damage in the upper wafer created by conventional rough and fine grinding is on the order of 10 um deep.
It is an object of the invention to produce a high quality edge finish on reworked edges of bonded wafers.
It is another object of the invention to minimize subsurface damage in the processing of bonded wafers.
It is another object of the invention to improve the process yield of bonded wafers.
It is another object of the invention to reduce the depth of edge damage in a processed upper wafer of a pair of bonded wafers to levels below 1 um.
Briefly, the invention provides an apparatus for polishing a bonded wafer that includes a support; a body of impact absorbent material mounted on the support; and a backing mounted on the body and having two surfaces disposed at an angle to each other and on an opposite side of the backing from the body and the support. In addition, the apparatus includes means for positioning a polishing tape on the surfaces of the backing whereby a first forward surface is positioned for polishing a peripheral edge of a top wafer of a pair of bonded wafers. Typically, this means is able to move the polishing tape relative to and along the two surfaces of the backing to present fresh polishing media, for example, in a manner as described in pending U.S. patent application Ser. No. 09/740,154 filed Dec. 19, 2000.
In a case where the bonded wafers are disposed on a horizontal plane, the apparatus positions the second or trailing surface of the backing at an acute angle relative to a horizontal plane, for example, in the range of from 3xc2x0 to 45xc2x0.
The apparatus also includes means for directing a coolant into an area under the polishing tape between the two angled surfaces of the backing for removing debris from between the bonded wafers and the tape.
The apparatus also includes means for oscillating the backing and tape thereon in at least one of a vertical plane and a horizontal plane during polishing of the top wafer or polishing of the edge of the handling wafer as well as means for moving the backing in a plane to allow the polishing tape on the trailing surface of the backing to polish a ledge on the handling wafer.
The invention also provides a method of polishing a bonded wafer including a handling wafer and a top wafer bonded to the handling wafer, the top wafer having a lesser diameter than the handling wafer and the handling wafer having an exposed ledge extending beyond the top wafer.
In accordance with the method, the pair of bonded wafers is rotated about an axis perpendicular to the top wafer while a backing having two surfaces disposed at an angle to each other is positioned with the forward surface opposite a peripheral edge of the rotating top wafer. In addition, a polishing tape is positioned on the forward surface of the backing in facing relation to the peripheral edge of the top wafer and the top wafer and backing are moved relative to each other to bring the polishing tape into polishing contact with the peripheral edge of the rotating top wafer.
In addition, during polishing, a coolant is directed into an area under the polishing tape between the two surface of the backing for removing debris.
The apparatus may also be employed to remove the bonding layer between the top wafer and the handling wafer. To this end, the polishing tape between the two angular surfaces of the backing are brought into contact with the bonding layer between the top wafer and the handling wafer to remove portions of the bonding layer therebetween. This step may be performed when only a portion of the peripheral edge of the top wafer is polished, e.g. in the vicinity of the bonding layer, or when the entire peripheral edge is polished.
The apparatus may also be employed during a polishing operation to polish the ledge on the handling wafer. To this end, the handling wafer and backing are moved relative to each other to have the polishing tape between the forward surface and the trailing surface of the backing polish the ledge as the handling wafer and backing are being moved relatively away from each other.
The apparatus and method provides a high quality edge that minimizes subsurface damage. This, in turn, improves the process yield and quality of a subsequent thinning process that reduces the top wafer to microns of thickness.